The present invention relates to a data transmission circuit and, more particularly, to a telephone data transmission circuit for establishing a data path for the bi-directional transmission of data between first and second locations over a standard telephone line.
There are many applications in which it is desirable to transmit data bi-directionally between a first location and a second location over a standard telephone line which is also used or shared by a telephone subscriber for making and receiving normal telephone calls. By way of example, the data at the first location may originate with a local data processing unit such as a microprocessor and be transmitted over the telephone line to associated remote apparatus at the second location. Data from the second location may also be transmitted over the telephone line to be used as desired by the data processing unit at the first location. In order to perform the above data transmission operations, especially since a subscriber may also use the same telephone line for making and receiving normal telephone calls, it is necessary that the telephone not be in use, that is, that the telephone be in an on/hook condition. It is necessary, therefore, that the on/hook condition of the telephone be first determined prior to any communication of data between the local and remote locations. However, in detecting the on/hook condition, it is generally desirable, in order to comply with government regulations relating to the connection of equipment to telephone networks, that the equipment used to detect the on/hook condition be isolated in some fashion from the telephone line. The present invention is directed to a data transmission circuit employing a simple, low-cost telephone status monitor circuit capable of satisfying the above-mentioned isolation requirements while also providing indications of on/hook conditions of a telephone for utilization by the data transmission circuit in establising bi-directional communication between local and remote locations.